This invention relates to electrical testing equipment and methods, and more particularly to a method for non-contact testing of small electrical conductors as on printing circuit boards or integrated circuit devices.
The purpose of this device is to produce an electrically-conductive plume of ionized gas (i.e., a plasma) that can be used to probe a circuit without actually coming into physical contact with the circuit. This device is intended for use in electrical tests such as simple open/shorts testing of electrical circuits in an engineering or manufacturing environment.
Methods previously used for performing electrical tests on circuit boards and the like have often employed some kind of physical contact, such as by a probe physically moved by an X-Y mechanism, or a so-called bed-of-nails. As the size of features on the electrical device have decreased, the likelihood of damage as a result of testing has greatly increased to the point where traditional probing methods simply cannot be used on some devices.
To circumvent the problem of physical damage by an electrical probe, several methods of no-contact testing have been developed. On method previously investigated was the use of an electrically-conductive, laser-induced plasma. For example, in my U.S. Pat. No. 4,970,461 for a "Method and Apparatus for Non-Contact Testing of Electrical Circuits," there is disclosed testing equipment where a laser is directed to selected spot on a circuit board held in a gas-filled chamber, with an electric field produced in the area of the target. A charge is built up on a node of the circuit board due to laser-induced discharge across the field, and when the field is reversed, a visible plasma is created as the charged node of the circuit is discharged. Continuity can thus be visually checked by observing which conductors are glowing.
Another method of non-contact testing using a laser to charge nodes on a circuit board is disclosed in U.S. Pat. No. 5,017,863, issued to Richard I. Mellitz, assigned to Digital Equipment Corporation. A scanned laser beam impinges on a photoemissive grid mounted close to the board under test, and electrons are emitted from the grid. A field between the grid and the board causes electrons to be attracted to the conductors of the board, and the amount of charge transferred can be measured to determine the amount of charge needed for a given node, giving an indication of opens or shorts.
Other methods of non-contact testing have used a scanned electron beam. These methods are particularly well adapted for scanning semiconductor integrated circuit devices, but require an evacuated chamber to hold the entire assembly and are complex and expensive, as well as being unwieldy in a production environment for large devices such as printed circuit boards.
A major drawback of prior methods of non-contact testing has been the inability to reliably test features smaller than about 25-mils. In order to decrease the size of the plasma, a prior attempt has made use of a plasma "torch" implemented by a small ceramic chamber in which a high voltage electrical discharge caused the formation of the plasma. Two permanent magnets on the chamber would interact with the residual sheath current in the plasma to electromagnetically force the plasma through a small orifice in the chamber. This orifice would shape and confine the size of this plasma. Unfortunately, the plasma produced by this "torch" is very noisy electrically, and so is of little use.